The development of quantum mechanics, nuclear physics and associated chemistry which occurred in the 1930's led James Chadwick (Nature 129:312, 1932) to the discovery of the neutron in 1932. Studies of the interactions of neutrons with a variety of materials uncovered the phenomenon of neutron scattering by elastic collisions (J. R. Dunning et al., Phys. Rev. 47:325, 1935) with atomic nuclei, especially the proton of the H-atom. Capture of slow (or thermal energy) neutrons by certain nuclei was disclosed in Fermi et al., Proc. Roy. Soc. London, 146:483 (1934) and the disintegration of other specific nuclei by interaction with thermal neutrons was disclosed in J. R. Dunning et al., Phys. Rev., 48:265 (1935). By 1935 a mass of experimental information had been collected, from which it was apparent that the ability of an atomic nucleus to capture a neutron was related not to the mass of the target nucleus, but to the actual structure of that nucleus. The concept of nuclei having a characteristic effective cross-sectional area, expressed as units of 10.sup.-24 cm.sup.2 known as barn units, was introduced with this early work. The effective nuclear cross section of boron for neutron capture was known to be exceptionally large while boron's neighbors in the periodic table, nitrogen and carbon, exhibited nuclear cross sections which were comparatively quite small.
Taylor, Proc. Roy. Soc., A47:873 (1935) described the capture of thermal neutrons by .sup.10 B nuclei followed by the production of .sup.4 He.sup.2+ (.alpha.-particles) and .sup.7 Li.sup.3+ with about 2 MeV of kinetic energy distributed between these two heavy ion products. It was also determined (Taylor, supra) that the translational range of the product ions was particularly short; about 7.6 .mu.in photographic gelatin and 1.1 cm in air. Consequently, the lithium ion and the .alpha.-particle products were short-lived, energetic species capable of imparting immense local damage to organic materials through ionization processes.
Gordon L. Locher of the Bartol Research Foundation of the Franklin Institute in Philadelphia, Pa. noted the potential medical applications of neutrons and boron neutron capture, in Am. J. Roentgenol. and Radium Therapy, 36:1 (1936). Locher's concept invoked the simple boron neutron capture reaction as the basis of a binary therapeutic method wherein a .sup.10 B nucleus contained in a compound which specifically localizes in tumor reacts with a thermal neutron to produce the energetic cytotoxic reaction products, an .alpha.-particle and a lithium ion. In this process no radioactive materials are involved and the therapeutic process may be modulated by controlling the supply of neutrons to the tumor site.
The two necessary components of the boron neutron capture (BNC) process, a controllable source of low-energy neutrons with a usefully high flux and suitable boron compounds for tumor localization were unknown in 1936 and Locher's concept remained prophetic until nuclear reactors were available to support an initial experimental test using thermal neutrons. This initial event did not occur until 1954 when Sweet, Farr and their coworkers (M. Javid et al., J Clin. Invest., 31:603 (1952); W. H. Sweet, N. EngL. J. Med., 245:875 (1951); W. H. Sweet and M. Javid, J. Neurosurg., 9:200 (1952); L. E. Farretal., Am. J. Roentgenol, 71:279 (1954); J. T. Godwin et al., Cancer, 8:601 (1955)) treated human brain tumors (glioblastoma multiforme) using .sup.10 B-enriched borate as the .sup.10 B target species in terminal patients. In these first experiments, BNCT was applied to the problem of killing malignant glioma cells which remained at the tumor site following normal surgical procedures.
The boron neutron capture (BNC) reaction obtained with thermal, 293K (0.025 eV), neutrons may be represented as shown in Equation (1): ##STR1##
The .sup.11 B nucleus is incapable of undergoing a BNC reaction while the effective nuclear cross section of .sup.10 B is 3837 barns.
Two other nuclides, .sup.1 H and .sup.14 N, are abundant in tissue and participate in important neutron capture side-reactions which occur during BNCT and thus contribute important doses of background radiation to the subject. These two neutron capture reactions play a role, not because of enhanced nuclear cross sections of the target nuclei, but due to their very high concentrations in tissue. As disclosed in H. Hatanaka, Boron-Neutron Capture Therapy for Tumors (H. Hatanaka, Ed.), Nichamura Co. Ltd., Nigata, Japan, p. 5 (1986), the neutron capture reactons of .sup.1 H and .sup.14 N are as shown in Equations (2a) and (2b), respectively: ##STR2##
The passage of a neutron through hydrogen-rich media, such as tissue, results in the slowing and scattering of these neutrons by collisions with nuclear protons of the H-atoms. Occasionally, a slowly moving neutron will be captured by such a proton and produce a deuteron accompanied by characteristic gamma radiation which contributes to the total radiation dose. In another competing capture reaction, the nitrogen atoms available in tissue may capture a low-energy neutron and generate .sup.14 C and an 0.63 MeV (kinetic energy) proton. The kinetic energy imparted to the .sup.7 Li.sup.3+ and .sup.4 He.sup.2+ ions derived from the BNC reaction and that similarly associated with the proton and .gamma.-photons, produced as shown in (2a) and (2b), is transferred to the surrounding media. Since all of these energetic nuclear reaction products, with the exception of the .gamma.-photons, are heavy particles, this kinetic energy transfer is rapid and takes place along a very short path length. The rate of linear energy transfer, LET, of these particles is characteristically high and the immense energy of these reactions is therefore deposited in a very small volume. As an example, the .sup.7 Li.sup.3+ and .sup.4 He.sup.2+ ions generated in the BNC reaction generate ionization tracks about 0.01 mm long or the equivalent of approximately one cell diameter. Thus, the high LET characteristic of particles produced by nuclear reactions which occur within tissue are especially lethal to affected cells due to the high density of deposited energy.
Ideally, those cells which carry large numbers of .sup.10 B nuclei are subject to destruction by BNC while neighboring cells that are free of .sup.10 B are spared, save for the contribution of the background .sup.1 H(n,.gamma.).sup.2 H and .sup.14 N(n,p).sup.14 C reactions. In order for the delivery of .sup.10 B to tumor cells to achieve this desired effect in BNCT the selectivity of boron delivery to tumor versus normal tissue, which is subject to neutron irradiation, should be as great as possible. In addition, the actual concentration of .sup.10 B in tumor must be sufficiently high to offer a localized binary therapeutic effect well above the background radiation dose delivered by the .sup.1 H(n,.gamma.).sup.2 H and .sup.14 N(n,p).sup.14 C neutron capture processes shown above. The minimum generally accepted .sup.10 B concentration necessary for effective BNC has been commonly believed to be between 10 and 30 .mu.g .sup.10 B/g tumor depending upon the precise location of the .sup.10 B with respect to vital components of the tumor cell structure. As the position of the .sup.10 B nuclei is changed from the external cell wall to the cytoplasm to the nucleus of the cell, the required concentration of .sup.10 B for effective BNCT decreases, as expected. Thus, cell wall-bound .sup.10 B might require 30 ppm or greater concentrations while .sup.10 B localized within the nucleus of the tumor cell might only require a concentration of 10 ppm or less. An additional factor is the steady state concentration of thermal neutrons in the targeted volume of tissue since very low neutron intensities require proportionally longer irradiation times to produce the required number of BNC events for effective therapy.
Due to the necessity of attaining base levels of tumor cell associated boron, attempts have previously been made to enhance the delivery of boron to tumor cells to obtain sufficiently high concentrations of boron in the cells for effective boron neutron capture therapy, while leaving relatively low amounts of background boron that can result in bystander cell or tissue damage upon neutron irradiation. For example, some tumor cell targeting strategies have employed the use of boron compounds with some natural affinity for tumors, such as 4-(dihydroxyboryl)phenlyalanine (BPA) or the mercaptoundecahydro-closo-dodecaborate dianion (B.sub.12 H.sub.11 SH.sup.2- ; BSH), or the attachment of boron-containing species to other molecules such as porphyrins. In addition, it has previously been proposed to conjugate .sup.10 B enriched boron to antibodies specific for tumor associated antigens to enhance the therapeutic effectiveness of BNC therapy. See, for example, D. Goldenberg et al., "Neutron-capture Therapy of Human Cancer: In vivo Results on Tumor Localization of Boron-10-Labeled Antibodies to Carcinoembryonic Antigen in the GW-39 Tumor Model System," Proc. Nat'l Acad. Sci. 81:560-563 (1 984); R. Barth et al., "Conjugation, Purification and Characterization of Boronated Monoclonal Antibodies For Use In Neutron Capture Therapy," Strahlenther. Onkol. 165(2/3):142-145 (1989); S. Tamat et al., "Boronated Monoclonal Antibodies for Potential Neutron Capture Therapy of Malignant Melanoma and Leukemia," Strahlenther. Onkol. 165(2/3):145-147 (1989); R. Abraham et al., "Boronated Antibodies for Neutron Capture Therapy," Strahlenther. Onkol. 165(2/3):148-151 (1989); A. Varadarajan et al., "Novel Carboranyl Amino Acids and Peptides: Reagents for Antibody Modification and Subsequent Neutron-Capture Studies," Bioconjugate Chem. 2(4):242-253 (1991); and R. Paxton et al., "Carboranyl Peptide-Antibody Conjugates for Neutron-Capture Therapy: Preparation, Characterization, and In Vivo Evaluation," Bioconjugate Chem. 3(3):241-247 (1991). Alternatively, it has been proposed to incorporate boron agents into liposome vesicles to provide an extended circulation lifetime for the agents, to protect the agents from attack by normal physiological agents and to reduce potential toxic side-effects. Certain liposomes also target specifically to neoplastic tissues. See, e.g., "Model Studies Directed Toward The Boron Neutron-Capture Therapy of Cancer: Boron Delivery To Murine Tumors With Liposomes," Proc. Natl. Acad. Sci. USA, 89:9039-9043 (1993).
Despite the advances that have been made in boron delivery methodology, the problems of low circulation lifetime, relatively low tumor specificity, less than optimal liposome composition, potentially toxic side effects and/or less than optimal cell membrane interactions have prevented these methods from achieving the highly specific delivery of boron to target tumor cells in therapeutically desirable concentrations. Thus, a strong need exists in the art for new and improved vehicles for the delivery of boron to target tumor cells for boron neutron capture therapy and imaging purposes.
It is also known that certain hormones bind to and activate specific intracellular receptors to alter the pattern of gene activity within cells. In the hormonal regulation of gene activity, a ligand (which can be a hormone or a synthetic analog) moves into a cell by facilitated diffusion. Once inside the cell, the ligand binds to its intracellular receptor to form a ligand/intracellular receptor complex that induces a change in the shape of the receptor and activates the receptor to carry out other functions within the cell. It is generally believed that the ligand/intracellular receptor complex recognizes and binds to specific short sequences of DNA within the control region of hormone-responsive genes, thereby mediating gene activity. Although much remains to be learned about the specifics of such mechanisms, it is known that exogenous inducers such as hormones modulate gene transcription by acting in concert with intracellular components, including intracellular receptors and discrete DNA known as hormone response elements. More specifically, it is known that hormones like the glucocorticoid, sex and thyroid hormones enter cells by facilitated diffusion. It is also known that the hormones then bind to specific receptor proteins, thereby creating a hormone/receptor complex. The binding of hormone to the receptor is believed to initiate an alosteric alteration of the receptor protein. As a result of this alteration, it is believed that the hormone/receptor complex is capable of binding with high affinity to certain specific sites on the chromatin DNA. Such sites, which are referred to in the art by a variety of names, modulate expression (transcription of RNA) of nearby target gene promoters. Unlike protein or peptide therapeutics, natural and synthetic ligands for intracellular receptors are small organic molecules sharing many of the attractive properties of pharmaceutical drugs, including suitability for oral or topical administration. The receptors for the non-peptide hormones are closely related members of a superfamily of proteins. This protein superfamily of receptors has been called intracellular receptors because the receptors are located inside target cells, unlike the receptors for neurotransmitters and protein or peptide hormones and growth factors, which are located in the plasma membranes of cells.